The present invention relates generally to radar systems and in particular to high precision range measurement techniques used in radar systems.
Electronic Warfare (EW) generally relates to military action involving the use of electromagnetic and directed energy to control the electromagnetic spectrum or to attack the enemy. The three major subdivisions within EW are Electronic Attack, Electronic Protection and Electronic Warfare Support. Electronic Attack (EA) is the division of EW involving the use of electromagnetic or directed energy to attack personnel, facilities or equipment with the intent of degrading, neutralizing or destroying enemy combat capability.
EW Systems aboard aircraft have long been used to protect the aircraft by denying a threat radar system the ability to track the target aircraft and guide launched missiles to intercept it. In a typical operation, a threat radar system tracking a target aircraft transmits RF signals, such as a sequence of pulses, aimed at the target. The surface of the target reflects a portion of the incident electromagnetic energy back toward the threat radar antenna where the pulse echo is detected, allowing the radar system to determine the target""s range, angle and velocity. Based on this information, the threat radar system can launch and guide a missile to intercept the target.
To effectively counter such radar systems, electronic warfare (EW) systems located on target aircraft attempt to interfere with threat radar signals by generating electronic counter measure (ECM) signals designed to confuse, mislead or overwhelm the tracking functions of the threat radar receiver. One technique used in the EW systems to interfere with the operation of threat radar system is to generate an ECM signal that is out of phase and inverted in amplitude as compared to the signal reflected by the target and received by the threat radar system. This technique is known as a Cross-Eye. Such an inverted signal is than targeted to the receiver of the threat radar to basically nullify the signal reflected by the target and received by the threat radar. To accomplish such electronic counter-measure, the EW system on the target aircraft first accurately determines the phase of a radar signal arriving at two spatially separated distant antennas located on the aircraft. Based on the phase difference of such received RF signal, an EW system can generate and transmit an ECM signal via the same two antennas towards the threat radar system. Such signals can be made to arrive on the threat radar antenna aperture 180 degrees out of phase and with inverted amplitudes.
In conventional EW systems, the two antennas receiving the RF signal from the threat radar are located on a single aircraft. When these antennas on a single platform are used to generate and transmit the interfering signal that is 180xc2x0 out of phase and inverse in amplitude compared to the reflected signal, such amplitude and phase inversion has to be very precise for such ECM system to work effectively. The errors tolerable in phase and amplitude inversion are directly proportional to the separation distance between the two platforms. In the conventional approach, where these antennas are on a single platform and the available separation distance is very short, the tolerable error is so small that the signal parameter values must be matched to within a few tenths of one degree in phase and within a few hundreds of one dB in amplitude. Such low error levels can not be practically achieved, thus making conventional ECM systems of this type ineffective, or at least sub-optimal.
Since both tolerable error values are directly proportional to the length of an imaginary line connecting the antennas, the higher the separation distance between the two antennas, higher the amount of tolerable errors. One solution to increase the tolerable error values is to increase the distance between the antennas. In a situation where two separate airborne platforms such as aircraft or Unmanned Airborne Vehicle (UAV) are available as mounts for the EW antennas, almost any length of baseline can be made available. The increased length allows for higher error tolerance that need be achieved in the matching of the phase and amplitudes of the signals generated by the EW antenna system. It is practically feasible to achieve phase and amplitude inversion with such higher error tolerance, which makes such systems practically useful.
However, to exploit the potential of a longer baseline between antennas requires a solution to the following problem. When an EW system mounts the antennas that generate the ECM signals on two different aircrafts, the local oscillator (LO) signals driving both of these antennas need to be either of same phase or of a known phase difference value so that any resultant errors may be compensated. If not compensated, the resultant phase errors between the signals of the two local oscillators appear as time delays in the radar signal. To take advantage of higher error tolerances available with the use of a longer baseline, a solution to this problem of phase errors between signals from two local oscillators mounted on different UAV platforms must be found.
Other problems and drawbacks also exist.
An embodiment of the present invention discloses a system for precise measurement of range between two platforms, using the global positioning system (GPS) located on both of these platforms, a range-only radar (ROR) located on one platform and a repeater located on the other platform.
According to one aspect of the invention, an electronic counter-measures system is provided with antennas located on two platforms.
According to another aspect of the invention, local oscillators are provided on both platforms.
According to another aspect of the invention, the precise measurement of range is used to calculate phase compensation sequence for one of the local oscillator.
According to another aspect of the invention, a local oscillator on one platform is phase compensated based on the information received from the other platform.
According to one embodiment of the present invention, the platforms are located on airborne vehicles.
Accordingly, it is one object of the present invention to overcome one or more of the aforementioned and other limitations of existing systems for very high precision range measurement.
It is another object of the present invention to provide an electronic counter-measures system using antennas located on two platforms that uses a very high precision range measurement system.
It is yet another object of the present invention to provide an electronic counter-measures system that overcomes the problems enumerated above associated with antennas located on a single aircraft.
It is another object of the present invention to provide an electronic counter-measures system to overcome the problems associated with low tolerable errors when using antennas located on a single aircraft.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute part of this specification, illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention. It will become apparent from the drawings and detailed description that other objects, advantages and benefits of the invention also exist.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the systems and methods, particularly pointed out in the written description and claims hereof as well as the appended drawings.